The oxidation of carotenoid upon illumination at low temperature has been studied in Mn-depleted photosystem II (PSII) using EPR and electronic absorption spectroscopy. Illumination of PSII at 20 K results in carotenoid cation radical (Car+*) formation in essentially all of the centers. When a sample which was preilluminated at 20 K was warmed in darkness to 120 K, Car+* was replaced by a chlorophyll cation radical. This suggests that carotenoid functions as an electron carrier between P680, the photooxidizable chlorophyll in PSII, and ChlZ, the monomeric chlorophyll which acts as a secondary electron donor under some conditions. By correlating with the absorption spectra at different temperatures, specific EPR signals from Car+* and ChlZ+* are distinguished in terms of their g-values and widths. When cytochrome b559 (Cyt b559) is prereduced, illumination at 20 K results in the oxidation of Cyt b559 without the prior formation of a stable Car+*. Although these results can be reconciled with a linear pathway, they are more straightforwardly explained in terms of a branched electron-transfer pathway, where Car is a direct electron donor to P680(+), while Cyt b559 and ChlZ are both capable of donating electrons to Car+*, and where the ChlZ donates electrons when Cyt b559 is oxidized prior to illumination. These results have significant repercussions on the current thinking concerning the protective role of the Cyt b559/ChlZ electron-transfer pathways and on structural models of PSII.

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